Preheat oven to 350 degrees F. Line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, cornstarch and salt; Set aside. In the bowl of a stand mixer fitted with the paddle attachment, cream butter, brown sugar, and zest together, about 2 minutes. With the mixer running, slowly drizzle in syrup and extract; beat until incorporated.
Add flour mixture to butter mixture in three additions, beating at low speed just until combined. Fold in the nuts.
Pat dough into an 8″ square about 3/4″ thick on the prepared baking sheet. Bake 30-40 minutes, or until brown around edges and golden in the center. Sprinkle a thick layer of superfine sugar over the shortbread
While shortbread is hot, trim edges, then cut into 1-inch squares and let cool on the pan to room temperature.